High-voltage direct-current (HVDC) transmission projects started more than 50 years ago in the electric power industry. Initially, HVDC schemes used mercury arc valves for switching, then the thyristor became predominant. Today, insulated gate bipolar transistors (IGBTs) are used in voltage source converter technology.
Presently, semiconductor devices (thyristors and IGBTs) for HVDC applications use silicon material. Silicon has remained as the industry workhorse for power electronics applications for more than five decades. However, silicon-based converters have limitations:
Limited maximum blocking voltage, which requires series connection of many devices and multi-level converters
Limited maximum switching frequency, which requires large passive elements for filtering
Limited operating temperature, which requires significant cooling.
A new set of materials known as wide bandgap semiconductors are being developed to address the above limitations. These wide bandgap materials include silicon carbide (SiC), gallium nitride (GaN), aluminum nitride (AlN) and diamond, though the most promising ones in the near future are SiC and GaN. Wide bandgap materials have several advantages:
Wide bandgap and high thermal conductivity allow high-temperature operation with reduced cooling
High saturation current velocity gives high current density
High breakdown electric field increases maximum blocking voltage of devices
High breakdown electric field and electron mobility give lower specific resistance for a given blocking voltage.
Research is ongoing to develop high-voltage, high-current and higher switching frequency power electronic devices using wide bandgap materials. This wide bandgap development is at different stages depending on the material. SiC-based high-power devices have the potential to demonstrate a much higher level of efficiency than silicon devices because of their much higher breakdown fields (more than 10 times) and thermal conductivity (more than double). However, SiC is still limited somewhat by the associated control electronics. It can operate up to and above 400°C (752°F), but the associated gate dielectrics still decompose at ~200°C (392°F), and packaging also has yet to match the high operating temperatures of SiC.
GaN has been used in a wide range of production LEDs and semiconductor lasers, in part because of their direct bandgap. GaN also has electrical and thermal properties that match closely those of SiC. However, bulk GaN substrates have not been successfully developed at a production scale like SiC. A key component of modern high-efficiency, high-power devices could be realized by the marriage of SiC base structures, with epitaxially grown GaN gates for high-power, optically controlled devices.
EPRI has conducted an industrywide survey to assess the future technological developments in the materials for power electronics applications. The figure shows the future voltage-rating projections for different devices based on different materials from now through 2030. The maximum operating voltage of a silicon-based device may be around 10 kV until some new breakthroughs in technology occur. Though SiC-based devices can go up to 100 kV (10 times the silicon-based devices) theoretically, for practical manufacturing limitations SiC devices are projected to reach 60-kV levels by 2030. Though GaN devices have more potential to reach higher voltages in the long run, they are expected to reach 20-kV levels by 2030. Similarly, the current ratings of wide bandgap materials will also reach in the several kilo-amperes range in the future.
It is fair to say that wide bandgap materials can be operated at higher voltages, higher currents and higher temperatures with lower switching losses compared to silicon, though it has a long way to apply wide bandgap materials for HVDC applications.
Ram Adapa (firstname.lastname@example.org) is a technical leader in the transmission and substations area of the power delivery and utilization sector of EPRI. He is a member of CIGRÉ, a registered professional engineer and an IEEE fellow.